1. Field of the Invention
The present invention generally relates to an optical pickup apparatus capable of reading two or more types of recording media at different reading wavelengths, such as a DVD/CD compatible optical pickup apparatus, and more particularly, to an optical pickup apparatus which employs a semiconductor laser device comprising a one-chip laser diode for emitting two laser beams at different wavelengths.
2. Description of the Related Art
Conventionally, there have been extensively proposed DVD/CD compatible reproducing apparatuses which share a single optical pickup for reproducing CD and DVD. These compatible apparatuses are implemented as a DVD/CD compatible reproducing apparatus employing a one-wavelength bifocal optical pickup, a DVD/CD compatible reproducing apparatus employing a two-wavelength bifocal optical pickup, and so on.
When CD is compared with DVD in terms of the structure, DVD has a protection layer the thickness of which is approximately one half of the thickness of a protection layer of CD (0.6 mm). Therefore, assuming that a unifocal optical pickup is used to reproduce both the optical discs, when a light beam is converged so as to be optimal for an information recording surface of DVD, aberration such as spherical aberration occurs in the light beam, due to the protection layer of CD, through which the optical beam passes, having a lager thickness than DVD, so that the light beam cannot be converged optimally for an information recording surface of CD. In addition, since CD differs from DVD in the size of information pits formed thereon for recording, it is necessary to form beam spots of sizes optimal to the respective information pits on the information recording surface of CD or DVD in order to accurately read respective information pits.
The size of a beam spot is proportional to the ratio of the wavelength of a laser beam to the numerical aperture of an objective lens for focusing the laser beam on an information recording surface. In other words, with a fixed wavelength of a laser beam, the beam spot becomes smaller as the numerical aperture increases. Therefore, for reproducing CD and DVD with a unifocal optical pickup, when the numerical aperture is set, for example, to fit for the information pits of DVD with the wavelength of laser beam being fixed, a resulting beam spot is too small for the information pits of CD, causing distortion in a reproduced signal from the CD and hence difficulties in accurate reading. For this reason, DVD/CD compatible reproducing apparatuses employing a bifocal optical pickup are prevalent because of its ability of irradiating two laser beams for providing focal points at different positions on the same straight line and forming beam spots of appropriate sizes corresponding to the sizes of the respective information pits.
Some of such optical pickup apparatuses for use in DVD/CD compatible reproducing apparatuses require two light sources and an additional combining prism, resulting in a higher cost as compared with an optical pickup apparatus having a single light source. When a first light source irradiates from one surface of a first beam splitter, a second light source must irradiate from another surface of the first beam splitter perpendicular to the first light source, so that a larger space is required for positioning the optical system, and accordingly the optical pickup apparatus becomes larger in size.
The present invention has been made in view of the problem mentioned above, and an object of the invention is to provide an optical pickup apparatus which is capable of supporting two wavelengths and amenable to a reduction in size without employing a combining prism.
To solve the problem mentioned above, an optical pickup apparatus according to a first aspect of the present invention is capable of reading information on recording media read by laser beams at different wavelengths. The optical pickup apparatus comprises light emitting means integrally including a first light source for emitting a first laser beam, and a second light source positioned in close proximity to the first light source for emitting a second laser beam at a wavelength different from a wavelength of the first laser beam; and an optical system including a plurality of optical elements passed by both the first and second laser beams for forming a light path for leading the first and second laser beams to a recording medium and for leading reflected light reflected by the recording medium to light detecting means, wherein the light detecting means includes a first detector for receiving the first laser beam and a second detector positioned in close proximity to the first detector for receiving the second laser beam, and the first and second detectors are arranged at positions such that the first and second laser beams follow return light paths of different lengths from the recording medium to the first and second detectors.
According to a second aspect of the present invention, in the optical pickup apparatus, the first and second laser beams follow different light paths from the light emitting means to the light detecting means.
According to a third aspect of the present invention, in the optical pickup apparatus, the first and second detectors are set at positions at which the first and second detectors receive focal points of the return light of the first and second laser beams, respectively.
According to a fourth aspect of the present invention, in the optical pickup apparatus according to the first or second aspect, the optical system includes an astigmatism element for applying the return light with astigmatism, and the first and second detectors are set to receive the first and second laser beams at positions at which the return light presents a substantially circular beam cross-section.
According to a fifth aspect of the present invention, in any of the optical pickup apparatuses, the detecting means includes the first and second detectors on the same plane, and is inclined with respect to the reflected beams.
According to a sixth aspect of the present invention, in any of the optical pickup apparatuses according to the first to fourth aspects, the light detecting means includes the first and second detectors formed at different heights, and is oriented substantially perpendicular to the return light.
According to a seventh aspect of the present invention, an optical pickup apparatus is capable of reading information on recording media read by laser beams at different wavelengths. The optical pickup apparatus comprises light emitting means integrally including a first light source for emitting a first laser beam, and a second light source positioned in close proximity to the first light source for emitting a second laser beam at a wavelength different from a wavelength of the first laser beam; and an optical system including a plurality of optical elements passed by both the first and second laser beams for forming a light path for leading the first and second laser beams to a recording medium and for leading reflected light reflected by the recording medium to light detecting means, wherein the optical system includes an astigmatism element for applying the return light with astigmatism, the light detecting means includes a first detector for receiving the first laser beam and a second detector for receiving the second laser beam, configured such that light receiving surfaces thereof lie in the same plane, and the light detecting means is arranged between two positions at which return light of the first and second laser beams presents a substantially circular beam cross-section, with the receiving surfaces oriented perpendicular to the return light of the first and second laser beams.
According to an eighth aspect of the present invention, in any of the optical pickup apparatuses, the light emitting means comprises a one-chip laser diode which is formed with an electrode of one of the first and second light sources as a common electrode.
According to a ninth aspect of the present invention, in the optical pickup apparatuses, the light detecting means is configured to selectively drive one of the first and second light sources depending on the type of a recording medium to be read.